After poking around on the board for a while tracing out where things go - YES it does use a 555 to flash the mute LED. and muting is done by latching a 4066 QUAD bilaterall switch to disconnect the mic - its not just a bit of code in the controller.

I noticed on the board "Yeti digital rev8C" hmm I wonder - punch Yeti digital REV8C schematic into duck duck go and find what looks like someone has started to reproduce the board but allas they dont appear to have gotten far .

https://github.com/HDR/Blue-Yeti-Replacement-Boards/blob/master/Blue_Yeti_Digital_Rev8C/CM6400.pdf

Oh well back to grind stone.